Transponders, which are also referred to as tags or labels, are well known in the art and are designed to communicate with a reader which is also known as a base station. Usually, the reader sends a signal to the transponder. If the transponder is close enough to the reader, i.e. if the power level of the signal generated by the reader is high enough so that the transponder can detect this signal, then the transponder receives this signal and may send, in response to the received signal, a signal to the reader.
Published U.S. application for patent No. 2006/0055552 A1 discloses a tag sending signals which comprise an information identifying itself and an information about the power level at which it sends the signals. The tag sends different signals at different power levels to a plurality of relay devices located at pre-defined locations. The plurality of relay devices are in communication with a tracking system. Depending on the power level of the signals, fewer or more relay devices receive the signals. The location of the tag can be determined from the receptions of the lowest power level transmission that is received by plural relay devices.
In some applications, for instance, when using a reader transponder system for mobile commerce, it may be desired that the transponder can only communicate with the reader within a certain distance or that this distance is flexible.